Rocket Construction: the Cherokee-C

Last week I described the Semroc Cherokee-C, so now it’s time to start building.

The first step for me is always preparing the balsa wood. There’s about as many ways to do that as there are rocketeers, but the goal is to treat the fins so that you’ll get a nice, smooth finish once they’re painted. Back in the day, I treated my fins with sanding sealer (e.g. Aerogloss) – but that’s smelly, noxious, and not really very good at filling the balsa grain. These days, some people treat balsa parts with cyanoacrylate (superglue), for strength. Other rocketeers prefer papering the fins. I’ve had some success with that, but usually only if the fins don’t have curves.

My go-to balsa treatment is Elmer’s Fill-n-Finish, which is a sort of water-soluble paste that you can smear onto the fins and then sand smooth. It doesn’t add a lot of strength, but it does a very good job of filling in the wood grain without adding much weight. I usually do 2 or 3 coats of Fill-n-Finish, sanded with 320 and 800 grit sandpaper, and the result is very smooth to the touch. My ideal goal is to leave just a slight trace of wood grain, so that the real materials show through just a little bit.

You can sand the edges of the fins to round them or streamline them, but in this case I left them square. That’ll reduce the rocket’s performance by a small amount due to increased drag. My usual flying field is on the small side, so I sometimes build my rockets draggy so they’ll be a little easier to recover. And sometimes that’s still not enough.

The motor mount. The motor itself fits in the end to the right, while the left-hand end with the shock cord is farthest inside the rocket body. The piece at bottom right is the payload bay endcap.

My favorite part of building a rocket is putting together the motor mount. It’s hard to explain why, exactly. It’s not a particularly tricky step or anything like that. Maybe it’s because this is the heart of the machine, the place where all the heat and thrust is contained and transferred to the airframe.

The motor mount is just a couple of tubes, some centering rings, and a hook to keep the engine in place. That thicker tube on the front end is one of Semroc’s improvements. It fits snugly inside the body tube, and provides a layer of protection between the body tube and the hot air and embers from the motor’s ejection charge. It also has a couple of slots cut in it for mounting the shock cord, so you’ve got the engine and the recovery system all built into one compact assembly.

With the motor mount in place, the fins go on next. The slotted body tube makes that pretty easy. Plus, Semroc includes a cutout to help get all three fins at the right angle, although it didn’t fit very well with this particular fin pattern. The cutout only slipped a little way up the fins, so there was still a bit of play to worry about and align by eye.

This kit includes a small payload section at the front of the rocket, which was a fun little piece to put together. It’s made of a short section of body tube with a section of coupler tube glued to the bottom. A wooden circle glues in to seal the bottom, and a screw eye – one of only two metal parts in a typical rocket – attaches to the circle. The screw eye is attached to the other end of the shock cord and also the parachute.

The last step of assembly is gluing on the launch lug, which keeps the rocket pointed in the right direction until it gets up to speed and the fins can provide stability. And there it is, nice and sleek – but not quite finished. As a rule, I don’t fly my rockets naked (the rocket isn’t naked, I mean, not me… er, I mean I’m not naked either… nevermind!). We still haven’t had any good flying weather, but I’ll be sure to post some more pictures once this rocket has been painted, decaled, and launched.